SPI Group is a specialist electrical installation firm covering both schools and libraries together with social housing across England and Wales.
npower aims to deliver energy to homeowners and tenants in the social housing which SPI installs and maintains services in.
Kevin Miles, chief executive of npower, said the aim is to "grow the business still further".
He added: "I am delighted that we have been able to acquire such a well respected, professional and quality orientated business such as SPI. This is a major step forward for npower in growing its energy services business."
The energy provider recently completed its new multi-million pound environmental upgrade to Aberthaw Power Station.
npower installed flue gas desulphurisation equipment, which reduces the amount of sulphur dioxide emission created by burning coal.
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